Get rid of most uses of Val{} types

[jmxpearson]

According to this I've been killing performance by using Val types.

Currently, we have ExprDist{V <: Val} <: Distribution, but it's probably better just to find a way to define a unique typename for each ExprDist using a heuristic that can be relied on.

Also, the Val trick is used to dispatch update! on the method argument, but this is easier to fix.

[jmxpearson]

This is also used all over the place in _simplify_call to break up logic based on the type of function call being simplified. This is probably just a matter of refactoring.

Thankfully, the only things really broken (or hobbled by this are ExprDists, which don't fully work yet anyway.